Machine designers have long known of the necessity of maximizing their designs both in terms of specific output parameters as well as total overall efficiencies. This driving force is especially important in today's competitive environment where excess design cost, weight, and complexity are no longer acceptable. No place has this drive been more universally accepted than in the design and manufacture of hybrid stepper motors.
To reduce cost and weight in the design and manufacture of hybrid stepper motors it is important that no unneeded material be included in the motor design to keep down cost and weight. However, it is also important that enough material be included in the rotor and stator design of these hybrid stepper motors to fully utilize the flux available from the expensive permanent magnets which are integral to the design of these machines.
In the past, some hybrid stepper motor designers focused on maximization of single output parameters in an attempt to produce a superior motor. However, the focus on any one single output parameter neglects overall motor design, and may sacrifice cost, weight, performance, and efficiency of the total design. One such attempt at single output parameter maximization is illustrated in U.S. Pat. No. 5,283,486 issued to Kobori for a STEPPING MOTOR.
In the Kobori '486 patent the single output parameter which is attempted to be maximized is the output torque of the motor. However, focusing solely on output torque neglects the more important machine parameter of overall torque density of the motor. While "torque" is a single output parameter of the motor, "torque density" is a measure of overall motor design efficiency which takes into consideration the torque per unit volume of the motor. The single parameter optimization of Kobori '486 neglects the three dimensional nature of the magnetic circuit of a hybrid stepper motor, and therefore, cannot optimize the overall machine design. In its attempt to maximize the single parameter of output torque, Kobori '486 specifies that a ratio of the stator inside diameter to the stator outside diameter must be kept within a range of 0.62 to 0.64 for a two-phase hybrid stepper motor. However, specifying only the ratio between the stator inside diameter to the stator outside diameter in the design of the hybrid stepper motor wholly ignores other design parameters which may greatly impact the overall design.
As an example, the simplistic approach of Kobori '486 ignores rotor segment length as an important design parameter. Utilizing a rotor segment length which is too short will tend to under-utilize the expensive rotor magnet causing a loss in torque and torque density of the machine. On the other hand, making the rotor segment length too long will increase the inertia of the rotor with no additional benefit in torque, along with a sacrifice in torque density, acceleration capability, and torque at high speed because of increased motor inductance. The cost of taking this simplistic non-three dimensional view to hybrid stepper motor design as suggested by Kobori '486 (maximization of output torque while ignoring overall motor torque density) is, therefore, simply too great in view of the significant impact that other hybrid stepper motor design parameters have on the overall design and performance of these machines. As a result, the quest to maximize the output torque parameter of the stepper motor without considering the three-dimensional nature of the magnetic circuit may in fact result in a design which is unacceptably inefficient, having unneeded material or under-utilizing the expensive permanent magnet of a hybrid stepper motor.